40 MUTATION AND PLANT BREEDING 



Table 4. — Selectional Balance when Fitness Varies with Genotypic Frequencies. 



Type Frequency, Advantage from Disadvantage from 



/ encounter, w encounter, w 



A. Advantage or disadvantage proportional to/ of unlike phenotype 



Dominant D + H \ +1 R \ -t R 



Recessive R 1 + s(D + H) 1 - s(D + H) 



Equilibrium Stable Unstable 



B. Advantage or disadvantage proportional to/ of own phenotype 



Dominant D + H 1 + t(H + D) 1 - t(H + D) 



Recessive R \ -\- s R \ — s R 



Equilibrium Unstable Stable 



quency, may be found either through the general procedure outlined 



previously (Table 2), or by the method of Wright (15) and Lewontin 



(9). In such simple cases as listed in Table 4, however, an expeditious 



short-cut may be used. The phenotypic frequency and the selective 



fitness should be so balanced that the population remains unchanged. 



This condition obtains when the fitness values of the two phenotypes 



are equal, thus 



s 

 for cases of Table 4 A, / R - s(\—R), R = 



s+t 



t 

 for cases of Table 4B, s R = t(\-R), R = - 



s+t 

 where R = q 2 + Fpq with inbreeding, and R — q 2 without inbreeding. 

 The stability of these equilibrium values has been indicated in Table 

 4. The general principle which governs a large number of similar 

 cases is 



selective advantage when abundant > unstable equilibrium 



selective advantage when rare — — > stable equilibrium 

 This principle is not only important in studying selection within 

 a population, but it is also important in ecological studies which deal 

 with the equilibrium between different populations. 



F. Metrical Trait 



Consider a metrical trait (say, size) whose measurement is x. 



